This invention relates to FM stereo demodulation circuits, and more particularly to an FM stereo demodulation circuit in which a subcarrier signal and a composite signal are subjected to multiplication in the demodulation of a subsignal.
It is known in the art, in the demodulation of an FM stereo signal, to separate the right and the left channel signals by switching the composite signal with a square wave subcarrier signal of 38 KHz. FIG. 1 is a block diagram showing such a circuit system. An FM-IF (intermediate frequency) signal is converted into a composite signal by an FM detector 1. The composite signal is applied to a switching circuit 3 through a LPF (low pass filter) 2 adapted to remove unwanted components. A pilot signal of 19 KHz included in the output of the LPF 2 is extracted by a PLL (phase-locked loop) circuit 4. A square wave subcarrier signal of 38 KHz synchronous in phase with the pilot signal is employed as a switching signal for the switching circuit 3. Audio components, or the right and left channel signals, are separated and led out of the switch output. For this purpose, low-pass filters (LPF) 5 and 6 are provided.
The 38 KHz subcarrier signal used as the switching signal is a square wave as shown in FIG. 2(A). Therefore, the Fourier expansion of this signal is as follows: ##EQU1## where .omega.s is the angular frequency of the subcarrier. Thus, the frequency spectrum of F(t) includes not only the fundamental wave of 38 KHz but also odd-order harmonics of 114 KHz, 190 KHz and so forth, as shown in FIG. 2(B).
By switching the FM detection output with a switching signal F(t) having such a frequency spectrum, the two signals are subjected to multiplication. Since the pass band of each of the low-pass filters 5 and 6 is 0 to 15 KHz, then the detector output which appears at the stereo output due to the multiplication is as shown in FIG. 2(C). That is, in addition to the main signal (0-15 KHz) and the subsignal (38+15 KHz), signals (noise signals or nearby interference waves) of 114.+-.15 KHz, 190.+-.15 KHz and so forth are demodulated as outputs.
In order to eliminate the above-described difficulty, it is necessary to provide low-pass filters having large attenuations at 114 KHz, 190 KHz and so on. However, since 114 KHz is near the composite signal component, the use of the LPF detracts from the flatness of the delay characteristic and amplitude of the composite signal as illustrated in FIGS. 2(E) and 2(D), respectively. As a result, the stereo demodulation output is distorted or the separation characteristic suffers.